This invention relates to thermodynamic machines for many different applications, such as hot-gas engines for vehicular applications.
The rising oil prices and the gradual depletion of the world's oil supplies have made the development of high-efficiency engines a matter of great importance. The internal combustion engine which is nowadays most widely used as an automotive engine has far too low an average efficiency to be acceptable in a near future. For example, in private car applications, the common Otto engine or four-stroke carburetor engine usually has an efficiency of less than 10%.
As a consequence of the increasing car density in the world, the problems caused by engine emissions have also become increasingly prominent. In internal combustion engines, work is performed as a result of combustion effected inside the cylinders of the engines through ignition of fuel introduced into the cylinders. The fuel consequently has to satisfy certain specific requirements in order to produce the required work in a satisfactory manner through the combustion process, and the exhaust gases, partly on account of incomplete combustion and partly on account of the presence of various additives in the fuel, have a composition that is environmentally unacceptable (high contents of CO, NO.sub.x, hydrocarbons, lead, etc.).
These disadvantages of the present-day internal combustion engines have markedly increased the interest in hot-gas engines during the last few years. In hot-gas engines, gas trapped in a closed system is caused to act on one or more pistons, by being caused to flow to and from one or the other side of the piston and heated and cooled in different suitable sequential steps. Since in the heating step heat is transmitted to the gas from an external arbitrary heat source, the heating can take place in such a manner that the purest possible exhaust gases are produced. The hot-gas engine can operate at a higher efficiency than the so-called Otto engine, and since the heat is produced outside the cylinder or cylinders, such as by external combustion, it is also decidedly more environmentally acceptable and can be run on a large number of different fuels, stored thermal energy or concentrated solar radiation, etc.
Extensive development work on hot-gas engines, primarily of the so-called Stirling type, is currently being carried out in several countries, primarily in the U.S.A., Sweden, Holland and Germany. Studies in this field have been concentrated in the first instance on the so-called double-acting Stirling engine with four pistons in four cylinders. In Stirling engines, gas is transferred between a cold and a warm cylinder containing a moving piston, the transfer taking place via a regenerator and a heater. In the double-acting Stirling engine, the pistons in pairs of interconnected cylinders work in different stages of a work cycle. Thermal net efficiencies (mechanical net power output divided by total applied chemical heat power input) near 40 percent for stationary operating conditions have been demonstrated experimentally with such engines, and temperatures of around 750.degree. C. have then been used in the heater. Even higher efficiencies may be achieved if the materials can be made to withstand higher temperatures. For example, using ceramic materials likely to be available in the future, hot-gas engines of this type can probably operate at efficiencies of around 50 percent or more. The problems associated with the Stirling engines are numerous, however. Among them, mention may be made of problems related to the materials, manufacturing problems and fundamental power-regulating problems.
Automotive engines have to satisfy highly exacting regulating requirements. Preferably, the average efficiency in the case of a varying load profile should also be high. With currently known Stirling configurations it is possible to satisfy the requirement for quick-response regulation, but as a rule it is not possible to satisfy the requirement for high efficiency with partial loads and high average efficiency during the transient processes occurring especially in city driving, that is driving characterized by frequent stops and starts and speed variations. The most widely used method of varying the mean pressure of the working gas in the Stirling engine by means of a compressor and a separate pressure vessel is thermodynamically irreversible, whereby a mechanical net power is consumed because of the transient processes, i.e. the average efficiency of the engine is lower than that achieved in stationary operating conditions. The mechanical design will be complicated and the manufacturing price of the engine will probably be high. The difficulties associated with regulation of the power output are believed to be one major reason why a definite break-through has not yet been achieved for the Stirling engine.
Another type of hot-gas engine is that described in U.S. Pat. No. 3,698,182. In the hot-gas engine of this patent, cooled working gas in a closed container (plenum chamber) is conveyed in different sequences into, out of and between two chambers which are separated by a movable wall common to both chambers and placed between the chambers in the form of a linearly movable or rotary piston. The gas in one chamber, the primary chamber, is hot and the gas in the other chamber, the secondary chamber, is cold. During the period of increasing primary chamber volume and decreasing secondary chamber volume, there occurs at the beginning of the period injection of working gas into the primary chamber, and particularly towards the end of the period discharge, hereinafter termed exhaust, from the secondary chamber takes place. During the period of decreasing primary chamber volume and increasing secondary chamber volume, a transfer of gas from the primary to the secondary chamber occurs. At the time when the engine according to this patent was devised, the possibility of making the secondary chamber smaller than the primary chamber for purposes of power output regulation was not realized. This hot-gas engine has been an object of comprehensive development work for a great many years and in the course of such work, a method for regulation of the power output has been devised which permits high efficiency values even at partial load and transient processes.
The above-explained problem has been solved by constructing the thermodynamic machine according to the invention as set forth in the claims. The invention primarily aims at solving the power-regulating problems in conjunction with the hot-gas engine according to the aforesaid patent, but it is not fully inconceivable that the same regulation principle in one modified form or other may also be usable for other types of hot-gas engines.